1. Field of the Invention
This invention relates to the field of human dietary supplements, and more specifically to a novel yeast strain of the genus Saccharomyces boulardii sequela which has the ability to process certain metallic compounds into forms that are biologically active and useful in supplementing the human diet.
2. Background of the Invention
Metal supplementation of the human diet is generally recognized as an important area in the field of nutritional science. Although no "RDA" minimum daily intake requirements have been officially adopted, research has strongly suggested that absence of some minerals, particularly metallic minerals such as chromium, germanium, and selenium, can lead to improper functioning of the body's metabolic processes, and a host of diseases and disorders. For example, selenium has been implicated as a mineral that may affect a reduction in the incidence of certain cancers when ingested in amounts of about 50 .mu.g. [W. Blott et al., J. Natl. Cancer Institute, 85(18):1483-92 (1993); H. P. Leis, Int. Surg., 76:1-5 (1991)]. Chromium is believed to play a role in modulating the symptoms of diabetes. [G. Mahdi and D. Naismith, Ann. Nutr. Metab., 35:65-70 (1991)].
Nutritional supplementation of the human diet with metals, using inorganic or organometallic sources for the metals, has met with limited success. Safety and efficacy in the human metabolism of such metals has been questioned, since most inorganic forms of metals are know to have appreciable toxicity. For example, selenium is usually administered as selenium trioxide species--an agent which is extremely toxic. Trivalent chromium has been administered as inorganic chromium, typically chromium chloride or chromium acetate, or as organometallic species such as chromium picolinate or chromium nicotinate, but each of these forms of chromium has been shown to be toxic in one fashion or another, reflecting a major shortcoming in their use in nutritional supplements. In addition, inorganic forms of such metals generally have a low bioavailability, making their use in nutritional supplements questionable.
Supplementation of the human diet with metal-enriched yeast products has been sought as an alternative. Potential advantages of administering a metal derived from yeast as a nutritional supplement over non-yeast derived forms are that the toxicity of the metal will be lower in yeast-derived organometallic compounds, and that since yeast-derived organometallic compounds are more soluble, such compounds will be better metabolized by the human body. However, a drawback of using yeast-derived metallic material as a food supplement is that rather large amounts of yeast are usually needed in order to acquire the proper dosage, since the levels of metal ion produced in yeast typically range from about 500 to 2000 ppm (.mu.g/g)--levels which would require ingestion of large amounts of yeast if the nutritional benefit of the organometallic compounds is to be realized.
3. Description of the Prior Art
Skogerson, U.S. Pat. No. 4,348,483, discloses a method for producing chromium-enriched yeast by incubating yeast in an aqueous solution of inorganic chromium salts, such as CrCl.sub.3, with growth media. Although the chromium content of the yeast is about 500 to 1000 ppm, the process entails, to a large degree, internalization of chromium without metabolization of the inorganic chromium salt so that there is a high probability that a significant, if not all the chromium found in the chromium-enriched yeast is inorganic chromium. Hence, to the extent that the chromium produced in this process is merely trivalent chromium salts mixed with the structural material of yeast, the chromium produced using this method does not possess high bioactivity and is also toxic if ingested in high doses.
Szalay, U.S. Pat. No. 4,343,905, teaches a method of concentrating chromium in Brewer's yeast by cultivating the yeast in a broth where the source of trivalent chromium is from a mixture of chromium oxide and certain amino acids. Although this method yields yeast with an intracellular chromium concentration of about 2000 ppm, 80% of which is reported to as yeast-metabolized chromium, this method encounters difficulties due to the insolubility of the chromium oxide. Also, similar to U.S. Pat. No. 4,348,483, internalization without metabolization of the chromium oxide is highly probable, and will likely result in chromium oxide, known to have appreciable toxicity to the human system, remaining in its toxic form.
The isolation and selection of a particular strain of Vietnamese yeast for assimilating about 300 ppm selenium metal is known. [Dang Hong Thuy, et al., Tap Chi Duoc Hoc, (4):9-12 (1992)]. Assimilation of inorganic selenium by Brewer's yeast, when grown in the presence of 150 ppm selenium trioxide disodium salt, produced a yeast product having about 1000 ppm selenium, and where about 3% of this was inorganic selenium, about 30% was selenoamino acids, and the remainder was present as other yeast-derived organoselenium compounds. [Xie, L. et al., Weishenwu Xuebao, 30(1) :36-40 (1990)].
A method for preparing yeast-derived germanium has been taught by Komatsu, Japanese Patent 77-46138770420. However, this method involves the preparation of germanium yeast using a highly toxic form of germanium, GeO.sub.2, as the source of germanium for the feed and cultivation of the yeast. The major shortcoming of this method is it results in an appreciable content of non-metabolized GeO.sub.2 by-product. Since GeO.sub.2 is a highly toxic agent, this method is not useful as a dietary supplementation for the human diet.
There remains a need for novel yeast strains that metabolize metal ions to provide a non-toxic yeast product that provides a form of the metal that is highly metabolizable by the human system and that is of sufficient concentration so that the product is suitable for the commercial marketplace for nutritional supplements.